The process for producing pressure sensitive adhesive (PSA) coated labels currently used by the industry consists of applying a solvent based or water emulsion based PSA to a low energy surface (release sheet), drying the adhesive and transferring the dry adhesive to the label stock by laminating the adhesive coated release sheet to the back surface of the label stock. The face of the label may be printed by conventional printing technologies. The label is then die cut to predetermined dimensions.
To apply the label to a substrate, the label is first removed from the release sheet, the adhesive now bonded to the back side of the label and the adhesive coated side of the label pressed onto the substrate. In order for the adhesive coated label to cleanly debond from the release sheet, the surface energy of the release sheet must be significantly lower than the surface energy of the adhesive. For typical polyacrylate ester adhesives, the surface energy is about 28 to 35 dyne/cm. A preferred release sheet is a polydimethyl siloxane (silicone) coated web. Its surface energy is about 22 dyne/cm.
The application of solvent based PSA's to release sheets presented little problem to the industry because the surface tension of such adhesive solutions was about 20 to 25 dyne/cm, low enough to provide good wetting of the release sheet.
Driven by environmental pressures and the cost of organic solvent, the industry has increasingly turned to alternatives such as 100% solids PSA's and waterborne PSA's. Currently, water-borne adhesives are the adhesive of choice because 1004 solids PSA's lack the balance of adhesive properties required by the label industry.
However, the surface tension of conventional emulsion polymers is typically greater than 40 dyne/cm. Consequently, any attempt to coat such emulsions on low energy surfaces leads to rapid retraction (crawling) and/or cratering of the adhesive film.
While several formulating techniques have been used to resolve this problem, none are completely satisfactory. A resolution to this problem is needed by the industry especially since environmental regulations will significantly restrict the use of organic solvent based adhesives in the future.
Presently, the most often used approach to resolving the coatability problem of water based adhesives is to reduce the surface tension of the emulsion by the addition of so-called wetting agents. A typically used wetting agent is dioctyl sulfosuccinate. The minimum surface tension that can be attained using this wetting agent is about 30 dyne/cm. Its use will significantly reduce, but not completely eliminate, crawling and cratering.
Another approach is to increase the viscosity of the emulsion with suitable thickening agents. As the viscosity is increased the rate of retraction is reduced. If the viscosity of a typical emulsion adhesive is raised to about 3000 to 5000 centipoises (cps), the rate of retraction and crater formation is sufficiently reduced such that on high speed coating lines the wet adhesive is dried before cratering occurs.
A combination of the two approaches is more generally used. However, there are significant drawbacks to these approaches. While reducing surface tension and increasing viscosity can improve coatability, leveling is improved by increasing surface tension and reducing viscosity.
Another problem associated with the use of wetting agents is their effect on adhesive performance. Shear adhesion and tack are reduced in the presence of dioctyl sulfosuccinate. Moisture sensitivity also increases.
Still another problem associated with wetting agents is foaming. Foaming can lead to loss of control of dry adhesive deposit level. Foaming can produce housekeeping problems if adhesive foams out of the applicator pan. Dried foam can deposit fish eyes in the adhesive coating.
The introduction of defoaming agents can create another set of problems. Particles of defoamer can serve as nucleating sites for formation of craters in the wet adhesive coating.
Yet another problem associated with wetting agents and defoamers is their time dependent performance in adhesive emulsions. The wetting agent and/or defoamer can be adsorbed by the polymer particles, thereby altering the coating performance of the emulsion. This leads to product rework and/or scrap.
The ideal coater ready PSA emulsion would be one which would be suitable for coating low energy surfaces at a viscosity as low as 500 cps without requiring the use of wetting agents or defoamers yet still have a relatively high surface tension to insure good leveling.
U.S. Pat. No. 4,668,730 addresses the problem of coating on low energy surfaces. The patent discloses a process for making an emulsion polymer adhesive which is characterized by superior rheological properties. The process consists of conducting the emulsion polymerization in the presence of an aqueous solution of alkali soluble oligomer. The alkali soluble oligomer is prepared by solution polymerization of a mixture consisting of esters of acrylic and/or methacrylic acid, an ethylenically unsaturated mono- or dicarboxylic acid, an acrylic or methacrylic ester of a polyalkylene oxide and, optionally, hydroxyalkyl (meth)acrylate or (meth)acrylamide.
The disadvantages of such a process are that a separate solution polymerization using an organic solvent is required and that the solvent must be stripped from the oligomer before it can be used in the subsequent aqueous emulsion polymerization. Another disadvantage is that the emulsion polymerization can only be conducted under alkaline conditions (the oligomer is dissolved in water by converting it to the ammonium salt).
Other patents describing emulsion polymerization in the presence of an alkali soluble oligomer include U.S. Pat. Nos. 4,845,149; 4,923,919; 4,879,333; 4,820,762; 4,151,143; and G.B. 1,107,249.
JP 91/02,293 discloses a low viscosity PSA emulsion having good wettability to release papers. The process consists of acrylate ester aqueous emulsion polymerization under alkaline conditions in the presence of a polymerizable emulsifier. However, the reported viscosity, 4000 cps, of the latex, under industry standards, would not be considered low viscosity. Most label converters in the United States coat adhesives having a viscosity of 500 to 1500 cps.
U.S. Pat. No. 4,894,397 discloses a process for producing a stabilized, inverted core-shell polymer emulsion by preparing a hydrophilic polymer and contacting the hydrophilic polymer with a hydrophobic monomer characterized in that the pH of the inverted core-shell polymer emulsion is adjusted to dissolve the hydrophilic polymer. The hydrophilic first stage polymers are alkali soluble.
U.S. Pat. No. 4,923,919 discloses emulsion polymer pressure sensitive adhesives made by a second stage emulsion polymerization. of a first stage polymer at a pH of at least about 7. Frazee's first stage polymers are prepared by solution polymerization according to CA 814,528.
U.S. Pat. No. 4,468,498 discloses the production of water-insoluble particulate heteropolymers made by sequential emulsion polymerization in which a "core" of a polymeric acid is at least partially encased in a "sheath" polymer that is permeable to a volatile base adapted to cause swelling of the core by neutralization. It is suggested that the dispersions of the invention may be used as quick tack adhesives.